Field of the Invention
The present invention relates to an inorganic material film which is a component of a photomask blank used to fabricate photomasks used in the microfabrication of semiconductor integrated circuits, CCDs (charge-coupled devices), color filters for LCDs (liquid crystal display devices), magnetic heads and the like.
Description of the Related Art
In the field of semiconductor technology, among others, a microfabrication technique is an extremely important basic technology, and therefore, research and development has been conducted for the purpose of further refinement. In recent years, particularly, due to high integration of a large scale integration circuit, refinement of circuit patterns, wiring patterns, or contact-hole patterns for wiring between layers forming a cell, a request for microfabrication technology has been increased.
In the context of the above-described circumstances, even in the field of technology for photomask production to be used in the process for photolithography at the time of microfabrication, a technique for making it possible to draw fine and correct circuit patterns (mask patterns) has begun to be demanded.
In order to form more precise mask patterns, a high-precision resist pattern needs to be formed on a photomask blank. Generally, reduction projection is performed when forming a pattern on a semiconductor substrate by photolithographic technique. Accordingly, the size of the pattern formed on a photomask is approximately four times the size of the pattern formed on the semiconductor substrate. This does not necessarily mean that accuracy is relaxed, but high accuracy is still required.
In today's photolithography technical field, the size of a circuit pattern to be drawn is considerably smaller than the wavelength of light to be used for exposure. Thus, in the case of forming a photomask pattern with a just four-times larger circuit pattern, light interference or the like, which is generated under exposure, influences on transfer of an original shape. As a result, the original shape cannot be transferred onto the resist film of a photomask blank.
Hence, the photomask pattern may in some cases be processed into a shape more complicated than an actual circuit pattern, in order to relieve such an effect. The shape may be, for example, a shape subjected to optical proximity correction (OPC).
In general, when forming a photomask pattern, a resist film is formed on the surface of the photomask blank in which a light-shielding film is deposited on a transparent substrate, and a pattern is then drawn (exposed) on the resist film by an electron beam. Subsequently, after obtaining a resist pattern after developing the exposed resist film, the light-shielding film is etched by using this resist pattern as a mask to obtain a light-shielding (film) pattern. The light-shielding (film) pattern thus obtained is served as a photomask pattern.
In this case, the above resist film should be thinned depending on the degree of fineness of the light-shielding pattern. This is because, when forming a fine light-shielding pattern while keeping the thickness of the resist film, the ratio (aspect ratio) of the thickness of the resist film to the size of the light-shielding pattern becomes large and causes troubles of failed pattern transfer, falling down or peeling off of the resist pattern, or the like due to deterioration of the shape of the resist pattern.
As a material of the light-shielding film deposited on the transparent substrate, many kinds of materials have so far been proposed. Among them, however, a chromium compound has been practically used because of much know-how on etching, for example.
Dry etching of a chromium film is generally performed by chlorine-containing dry etching. In many cases, however, chlorine-containing dry etching has a certain level of ability to etch an organic layer. Accordingly, if the resist pattern is formed on a thin resist film when the light-shielding film is etched, this resist pattern undergoes a measurable degree of etching due to chlorine-containing dry etching. This adverse effect will result in a failure to precisely transfer an originally-intended resist pattern onto the light-shielding film.
A resist material having excellent etching resistance has been requested. However, the reality is that such a resist material has not been known yet. For such reasons, restudies are being made of light-shielding film materials having higher processing accuracy, in order to obtain high-resolution light-shielding (film) patterns.
An effort to improve the etching rate of a light-shielding film by allowing a chromium compound which is a light-shielding film material to contain only a predetermined amount of light element has been reported as a specific example of approach to the restudies of light-shielding film materials having higher processing accuracy (WO 2007/74806 A, JP 2007-33470 A, and the like).
WO 2007/74806 A discloses a technique for reducing resist film loss by using a material mainly containing chromium (Cr) and nitrogen (N) and having an X-diffraction peak of substantially CrON(200) to enhance a dry-etching rate of the light-shielding film.
In addition, JP 2007-33470 A discloses the invention of a photomask blank where the composition, film thickness, and stacked structure thereof are suitably designed to obtain desired transmittance T and reflectance R while attaining an increase in the rate of dry-etching by providing the light element with rich content of a light element and low content of chromium.
However, the technique as described above, where a light element is added to a chromium-containing compound to suppress a decrease in thickness of a resist film by increasing the dry-etching rate of a light-shielding film, has the following disadvantage:
When using a chromium-based compound as a light-shielding film material, the light-shielding film should not only ensure its improved etching rate but also ensure predetermined optical containing characteristics because the light-shielding film also serves as an optical film. However, the flexibility of the film design enough to simultaneously satisfy both demands is not always high.
Even if the chromium-containing compound is used as a film material for forming a hard mask to be used to process the light-shielding film rather than as a material for the light-shielding film, the range of amounts of light element that can be added to secure the functionality of the compound is limited naturally. The flexibility of film design is therefore not necessarily high, either.
For such reasons as described above, there is the desire for the provision of techniques to improve the etching rate of films composed of a chromium-containing compound by a procedure different from conventional procedures, such light element addition.
As one of the techniques, JP 2013-238776 A discloses allowing a chromium-containing compound to contain tin to improve the etching rate of a film.
It has become clear, however, that tin particle agglomeration may take place in such a tin-containing chromium-containing compound to such a degree as is discriminable at a surface roughness level. If this compound is formed into a hard mask film and dry etching based on oxygen-containing chlorine is performed to draw a fine pattern, etching progresses locally in such locations where tin particles are agglomerated. This can be a cause for defects or the degradation of pattern fidelity and is, therefore, undesirable.